Support "soft" feature-gating using a lint

Use it for feature-gating `#[bench]`

2 files changed, 14 insertions, 0 deletions

diff --git a/src/test/ui/feature-gates/bench.rs b/src/test/ui/feature-gates/bench.rs
new file mode 100644
index 00000000000..d8a09625e96
--- /dev/null
+++ b/src/test/ui/feature-gates/bench.rs
@@ -0,0 +1,4 @@
+#[bench] //~ ERROR use of unstable library feature 'test'
+fn bench() {}
+
+fn main() {}
diff --git a/src/test/ui/feature-gates/bench.stderr b/src/test/ui/feature-gates/bench.stderr
new file mode 100644
index 00000000000..25ddcc5e361
--- /dev/null
+++ b/src/test/ui/feature-gates/bench.stderr
@@ -0,0 +1,10 @@
+error: use of unstable library feature 'test': `bench` is a part of custom test frameworks which are unstable
+  --> $DIR/bench.rs:1:3
+   |
+LL | #[bench]
+   |   ^^^^^
+   |
+   = note: `#[deny(soft_unstable)]` on by default
+
+error: aborting due to previous error
+